1. Field of the Invention
The present invention relates to a color cathode ray tube used for home television sets, computer monitors, and the like.
2. Description of the Related Art
A color cathode ray tube (hereinafter referred to as a color CRT) has been broadly used for a variety of home and industrial apparatuses including television sets and computer monitors. An enhanced quality of images is always requested for such a color CRT. Especially, in recent years, there has been a strong need to realize a color CRT capable of producing images with high resolution and high color tone without distortion over the entire front surface of the color CRT.
Conventionally, a color CRT includes a glass bulb (hereinafter referred to as a bulb). The bulb has a curved panel made of glass (hereinafter referred to as a curved panel) and a phosphor screen is formed on the inner surface of the curved panel for emitting three colors, i.e., red, green, and blue. The bulb also includes a funnel which is attached to the curved panel by means of a glass adhesive so as to form the bulb. The inside of the bulb is in a high vacuum condition.
A curved shadow mask with a thickness of 0.1 to 0.3 mm having a number of apertures formed therethrough is disposed at a position close to the inner surface of the curved panel, facing thereto. The shadow mask is secured to a metal frame disposed inside the bulb so as to follow the curved profile of the inner surface of the curved panel.
The funnel includes a neck portion in the rear thereof where an electron gun is disposed for emitting electron beams. The electron beams emitted from the electron gun pass through the apertures of the shadow mask to reach the phosphor screen on the inner surface of the curved panel, allowing the phosphor screen to emit light.
If the distance between the shadow mask and the phosphor screen changes, the color tone of images produced on the phosphor screen also changes, resulting in a deterioration of the quality of the images. Accordingly, in order to obtain stable operating properties of the color CRT, the distance between the shadow mask and the phosphor screen should be kept constant irrespective of any change in the environmental conditions. However, in the above conventional color CRT where both the shadow mask and the phosphor screen are curved, it is difficult to precisely control the distance between two curved surfaces.
In general, only approximately 20% of the total emission of electron beams output from the electron gun are actually incident to the phosphor screen. The remaining electron beams are absorbed by the shadow mask, causing an increase in the temperature of the shadow mask and thus an expansion thereof. In the conventional color CRT where the shadow mask is curved along the curved profile of the inner surface of the curved panel, the shadow mask becomes deformed due to the thermal expansion so that it becomes closer to the inner surface of the curved panel, in other words, the phosphor screen formed thereon. As a result, the resultant images produced on the screen become deteriorated because of the expansion.
In order to obtain images with high resolution and high color tone, the shadow mask should be thin, and the pitch of the apertures formed in the shadow mask should be small. In the conventional color CRT having a curved profile, it is difficult to reduce the thickness of the shadow mask beyond a certain level because below such a level, a sufficiently large mechanical strength will not be obtained.
Further, in general, the pitch of the apertures formed on the shadow mask should be twice the diameter of the apertures so as to avoid mis-landing the electron beams on the phosphor screen. Also, in order to secure the process accuracy, the minimum diameter of the apertures should be at least four fifths of the thickness of the shadow mask. Under these limitations, it is difficult with the conventional curved shadow mask to decrease the pitch of the apertures to 0.2 mm or less so as to obtain images with high resolution and high color tone.
In order to overcome the above mentioned problems, a flat panel made of glass (hereinafter referred to as the flat panel) may be used instead of the curved panel. However, such a conventional panel has the following disadvantages.
The flat panel needs to be thick enough to resist a significantly large pressure difference between the inside and the outside of the bulb caused by the high vacuum so as to prevent the bulb from breaking. As the flat panel becomes thicker, the distortion of the images produced on the phosphor screen becomes greater due to the deflection of light through the glass. In some cases, only the outer surface of the conventional panel is made flat, although the inner surface of the conventional flat panel remains curved. This brings a non-uniform thickness of the flat panel and consequent difference of intensity of the transmitting light between the center portion of the flat panel and the peripheral portion thereof, resulting in a nonuniform luminance distribution of the images. Besides the above optical disadvantages, such a thick flat panel is disadvantageous in that the resultant color CRT becomes heavier.
As described above, the conventional flat panel fails in realizing a color CRT with high performance which can completely replace the aforementioned conventional color CRT having the curved panel.